The process of transferring cellular-based calls from one cell cite to another as the mobile or portable moves through the service area is known as a "hand-off." There are two forms of hand-off: hard and soft. A hard hand-off requires the connection to be broken in the original cell before it is made in the successor cell. In general, in a CDMA communication system, a hard hand-off occurs when different frequency bands are allocated between neighboring base stations (i.e., adjacent cells). A base station transmits a hand-off message to a mobile station to allocate a new frequency band. Then the mobile station terminates transmission of the original base station and tunes itself to the new frequency of the successor base station. Once the mobile station has sequentially received two good frames from the successor base station, it starts transmission thereto, so that a hard hand-off can be performed from one cell to the other cell. The hard hand-off methods in the prior art are mainly performed utilizing hardware assistance. For example, if one mobile station has been communicating with base station "A" at a predetermined frequency band and nears another base station "B", there are two approaches to hand-off between base stations "A" and "B". In the first approach, base station "B" uses the pilot signal strengths of the same frequency band as the base station "A". In the second approach, base station "B" uses another frequency band, different from the predetermined frequency band, by determining with a software program the necessary information when the mobile station becomes closer to base station "B".
Also, because inter-cell hand-off timing through the prior art software program is not exact, the hand-off is performed at a common frequency band at the center of a cell not at the boundary of the cell. Accordingly, in the overall CDMA system of the prior art, calls are concentrated in the common frequency band. Therefore, in the digital mobile communication utilizing CDMA a problem exists in the hand-off by such software program.
U.S. Pat. No. 5,649,000 discloses a method and apparatus for providing a CDMA-to-CDMA different frequency hand-off in a CDMA cellular telephone system. A cellular system transfers a call from a source base station to the target base station. A mobile unit measures the strength of pilot signals emanating from surrounding base station. In response to the result of the measurement, a different frequency hand-off operation is initiated by the mobile unit when all pilot signals are lower than a threshold or is initiated by a system controller with consideration being given to the frequency band occupation state of surrounding base stations and the strength information reported from the mobile unit. This patent, however, has a problem in that it requires assistance of hardware such as a controller, a data positioning device and a frequency synthesizer.
Conventionally, the general base station uses multiple frequency assignments (FA) such as frequency bands of 779 MHz, 738 MHz, 697 MHz etc., to communicate with the mobile station, but a small base station uses only one of the frequency bands, i.e., one FA, and cannot use a dummy pilot signal. Accordingly, if one mobile station which has been communicating with the general base station at a certain frequency band moves from the cell region of the general base station to the cell region of small base station that uses different frequency bands than the original frequency band, an abrupt cut in the call occurs because the hand-off cannot be performed. FIG. 1, shows a hand-off situation during which a mobile station (not shown) moves from a general base to a small base station. The solid line circles "A", "C" and "D" represent the general base station and the dotted line circle "B" represents the small base station. In this figure, assuming that the general base station "A" uses two FAs of 779 MHz and 738 MHz and the small base station "B" uses one FA of 779 MHz, if the mobile station has been communicating with base station "A" at 738 MHZ, the mobile station cannot communicate with base station "B" at the cell boundary of the base station "A" because the frequency cannot be set at 738 MHz when base station "B" is using 779 MHz.